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The Messenger spacecraft that sped past Mercury on Jan. 14 sent back pictures of a geological formation never seen before in the solar system: a central depression with more than 100 narrow troughs radiating out from it.

Called "The Spider" by scientists analyzing the trove of images and data coming back from Messenger, the puzzling feature is the kind of surprise that researchers live for.

"Messenger has sent back data near perfectly, and some of it confirms earlier understandings, and some of it tells us something brand-new," said principal investigator Sean C. Solomon. "The Spider is definitely in the category of something we never imagined we'd find."

Scientists were also surprised by evidence of ancient volcanoes on many parts of the planet's surface and how different it looks compared with the moon, which is about the same size. Unlike the moon, Mercury has huge cliffs, as well as formations snaking hundreds of miles that indicate patterns of fault activity from Mercury's earliest days, more than 4 billion years ago.

"It was not the planet we expected," said Solomon, of the Carnegie Institution of Washington. "It's a very dynamic planet with an awful lot going on."

Messenger passed by Mercury after a journey of more than 2 billion miles. It will swing by the planet twice more before settling into orbit around it in 2011.

Mercury is among the least understood planets because its proximity to the sun makes it hard to visit and to explore. Among the mysteries researchers hope to unravel is where and how Mercury was formed and the nature of the magnetic fields around it. Earth is the only other planet with such an active magnetosphere.

Solomon said clues into whether Mercury once orbited much farther from the sun, as theorized by many scientists, may emerge as the craft begins to orbit and conducts a chemical analysis of the surface.

Among the early findings is that a crater called Caloris is larger than researchers thought after the Mariner 10 spacecraft sent back the first images of the planet 33 years ago. Scientists now believe it is more than 950 miles wide. The Caloris basin, created by a long-ago asteroid strike, is home to "The Spider."